1. Field of the Invention
The present invention relates to voltage converters, and more particularly to a device for driving a power transistor of a voltage converter provided with a piezoelectric transformer and a voltage converter including such a device.
2. Description of Related Art
Switching voltage regulators provided with power transistors driven suitably by circuitry are generally known. In some cases the regulators include some piezoelectric transformers adapted to convert the applied electrical energy by exploiting a mechanical resonance.
The value of the output voltage in a piezoelectric transformer depends greatly on the value of the frequency of the input signal; in fact the piezoelectric transformer is adapted to transfer electrical energy only in a very narrow frequency band around its mechanical resonance frequency. With respect to the typical electromagnetic transformers, piezoelectric transformers present the advantages of having reduced sizes and weights and a high efficiency (on the order of 90%). For this reason it is used in different applications in which small size and high efficiency requirements are fundamental, as in the case of portable devices that are supplied from batteries.
FIG. 1 shows the electrical circuit equivalent for a piezoelectric transformer in the case in which the input signal is at the mechanical resonance of the transformer. A first series of components constituted by a capacitor Cd1 and a resistor Rc is arranged in parallel to a second series of components constituted by a resistor R, an inductor L, a capacitor C, and a voltage generator Vout/N (where N is the transformer ratio and Vout is the output voltage). A current I2 goes through the second series of components, and both the first series of components and the second series of components are connected between an input voltage Vin and ground. The circuit also includes a current generator I2/N and a capacitor Cd2 connected in parallel to one another and connected between the output voltage Vout and ground. The capacitor Cd1 generally has a high value and this causes the application of an input voltage Vin of the sinusoidal type for keeping the efficiency of the transformer high.
Normally a filtering inductor is placed upstream of the transformer in voltage converters that have piezoelectric transformers; the value of the inductor is selected according to the capacitor Cd1 and the mechanical resonance frequency.
FIGS. 2 and 3 show two circuit configurations of a switching DC/DC converter. In FIG. 2, an inductor L1 filters a square wave signal derived from the half bridge constituted by transistors M1 and M2, which are connected in series between a voltage Vin1 and ground and are driven by a driving device 5. The inductor L1 is connected to the non-drivable terminal common to the transistors M1 and M2 and to a piezoelectric transformer 1 that has the electrical circuit equivalent shown in FIG. 1. The output sinusoidal voltage signal of the piezoelectric transformer 1 is rectified by a rectifier circuit 2 and is applied to a load LOAD. A part of the voltage Vo applied to the load, that is the voltage R2*Vo/(R1+R2), is supplied to the inverting terminal of an error amplifier 3. The non-inverting terminal is connected to a reference voltage Vref. The output signal of the error amplifier 3 is supplied to a voltage controlled oscillator (VCO) 4 that causes the switching frequency to be sent to the driving device 5; the driving of the transistors M1 and M2 occurs with a duty cycle of 50%. The VCO 4 is normally set for operating in a frequency interval A that is monotone, fixed. and belongs to the transfer characteristic of the transformer, as shown in FIG. 4. The frequency is changed according to the desired output voltage. When using the converter shown in FIG. 2, the determination of the resonance frequency of the transformer, which may change based on the resistive value of the load and based on the operating temperature, is difficult and therefore the suitable setting of the VCO 4 is difficult.
FIG. 3 shows a DC/DC converter which differs from the circuit of FIG. 2 by the presence of a single MOS power transistor M3 having its source terminal connected to ground and its drain terminal connected to an inductor L2, which has in turn its other terminal connected to the input voltage Vin1. The driving device 6 drives the single transistor M3 with a duty cycle of 50%. In this circuit a quasi-sinusoidal signal is applied to the piezoelectric transformer 1 due to the resonance between the inductor L2 and the capacitor Cd1 of the transformer 1. The capacitor Cd1 is discharged before the turning on of the transistor M3 and before the consequent recharging of the inductor L2. In such a circuit, the efficiency of the transformer is linked to the switching frequency.